1. Field of the Invention
The present invention relates to an audio amplifier circuit and an audio IC having the same, and more specifically, relates to an audio amplifier circuit used in the fields such as a cellular phone, a portable terminal apparatus, a fixed phone and a note type personal computer, which is permitted to drive a speaker in accordance with the withstand voltage of a speaker to be connected or the maximum input power thereof and is suitable for a small size speaker.
2. Conventional Art
In the fields such as a cellular phone, a portable terminal apparatus, a fixed phone and a note type personal computer, since the circuit scale and the mounting space of an audio IC is limited, a small size speaker having a small output of about 0.5 W through 1 W is used. When a user drives these sorts of speaker for a long time with the maximum volume thereof, the voice coil of the speaker sometimes damages.
As a protection circuit for such speaker damage, conventionally, with regard to a high output amplifier, a circuit in which such as a fuse is provided in series between an output terminal and a speaker has been known. Further, as an over voltage protection circuit for such as a high output amplifier, a diode clump circuit provided in parallel with an output stage transistor is also known. Still further, JP (U)-B-7-35449 discloses an audio amplifier circuit provided with a diode limit circuit at an output stage.
Now, as a power amplifier for an audio amplifier circuit, one having a maximum output (an output power) of about 1.0 W is generally available as in a form of an IC, when using the same, a speaker to be connected having a maximum input watt more than 1.0 W is selected. For a speaker having a maximum input watt less than 1.0 W, respective output stage amplifier circuits have to be designed while matching with the withstand voltages or the maximum input powers of the respective speakers. Because, if not being matched, a possible damaging the speakers become high. However, a size of a power amplifier having a maximum output watt of about 1.0 W does not differ from one having about a few W as ICs and the mounting spaces thereof also substantially the same.
On one hand, in connection with such as a cellular phone, a portable terminal apparatus and a fixed phone, depending on the models thereof, either a small output or a large output is required. For this reason, several kinds of ICs for audio output amplifiers are required to be prepared depending on the audio output powers. When the production amount of the audio output amplifiers having a maximum output watt of about 1.0 W or less than that decreases, a problem arises to increase the cost of these ICs.